The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Filters that are used to detect narrow band transient signals often must operate in a signal environment to detect the transient signals (e.g., acoustic, radio frequency, etc.) where the center frequency of the transient signal is not known a priori, and where the transient signal is present in a broadband frequency spectrum being monitored by the filter. Often, the signal environment also includes a non-uniform, varying noise floor, and also long-duration signals such as continuous wave (CW) and broadcast signals (e.g., FM, TV). The objective is to detect the transient signals without providing false detections due to variations in the background noise level, and/or due to the presence of the long-duration signals. Conventional techniques often employ linear averaging filters to set detection thresholds, but such filters provide no discrimination based upon a signal's duration. Linear, averaging filters typically average the total energy of a signal within a particular frequency band. Thus, high-energy transient signals and long-duration, low-power continuous wave signals are treated the same by these linear types of filters.
Typically, to detect a transient signal, a power spectral estimate (PSE) of the ambient signal environment is computed, from which an adaptive detection threshold is derived via a linear, low-pass filter. This detection threshold is placed above the noise level so that the noise rarely exceeds the detection threshold. However, when the detection threshold is exceeded by the PSE, a transient signal detection is declared by the filter.
It would be highly desirable if the transient detection threshold of the filter could be set to remain above the background noise level, and also above the level of long-duration signals like continuous wave signals, so that neither background noise nor long-duration signals are detected by the filter as transient signals. It is also desirable that the threshold remain at a level consistent with (but slightly above) the ambient noise level, and below the level of any transient signals that occur, so that the transient signals will be detected.
Thus, what is required is a filter that allows its transient signal threshold to operate without being influenced (i.e., being changed in threshold level) by the signals which the filter is trying to detect, namely transient signals. This is a limitation with present day linear filters. Due to the nature of a linear filter, such as that described above, the detection threshold of such a filter is based upon the average signal power being detected by the filter, regardless of whether the signal is a continuous wave signal or a transient signal. Although one can reduce the affect the transient signals have on a linear, low-pass filter's output by lowering the cutoff frequency, doing so introduces the additional problem of long settling times required when setting the threshold level.
Thus, it would be highly desirable to provide a filter able to operate within a broadband ambient environment, in which transient signals reside, and which provides a scanning process which adapts to changes in the signal environment to detect transient signals without false detections from long-duration signals and/or variations in the background noise level of the ambient signal being monitored, and without having its detection threshold influenced by the transient signals.